Power tools typically employ a motor that imparts torque to a tool through a spindle. In the case of an electric drill, the motor spindle is coupled through a series of reducing gears to the chuck, which in turn holds the drill bit or other cutting/abrading tool, such as a hole saw, a grinding wheel or the like. Power screwdrivers as well a large rotary hammers work on a similar principle. In each of these cases, the function of the reducing gears or gear train is to reduce the rotational speed of the tool while increasing the rotational torque.
Power routers are somewhat different. The cutting tool of the hand-held router is typically direct coupled to the spindle of the motor. In this case, the full rotational speed of the motor is used without gear reduction to rotate the router bit at high speed. Reciprocating saw and jigsaws use yet another type of gear train that translates the rotational motion of the motor spindle to reciprocating movement.
Generally speaking, all of these power tools may suddenly encounter an impending kickback condition at which time the output torque rapidly rises because of local changes in workpiece hardness, workpiece binding, tool obstruction from burrs and so forth. For example, when drilling a hole with a power drill, some workpieces will develop burrs on the tool exit side of the workpiece. These burrs can engage the flutes of the drill bit, thereby causing a rapid increase in torque as the drill tries to break free. In some instances, the burrs may stop drill bit rotation, thereby causing a strong reaction torque that is imparted to the tool operator as the motor turns the tool in the operator's grasp (rather than turning the drill bit). This reaction is can be problematic if the operator is standing on a ladder and/or holding the tool over their head. A related phenomenon also occurs with power saws. These conditions are hereinafter generally referred to as kickback conditions, regardless of the particular power tool involved or the specific circumstance which give rise to the condition.
Therefore, it is desirable to provide an improved technique for detecting the onset of such kickback conditions in power tools.